Hi! I have these Ultegra 6700 shifters and a Deore 9 speed rear derailleur. They work fine, but they "lose adjustment" pretty easily. I'm thinking about upgrade into this Shimano GRX RD-RX400 (
Shimano Ultegra 6500 ST-6510 RIGHT/REAR 9 Speed STI Shifter, 7/10 Nice – EJRecyclery - Cycling New & Used
NOS Shimano Ultegra ST-R600 STI Shifters: 9 Speed Triple/Double - Non Series ST-6500/6510 - Front/Rear - Bike Recyclery
Shifters - Bar End - Shimano Dura Ace, 9 Speed – Rivendell Bicycle Works
Amazon.com: microSHIFT R859 2 x 9-Speed Flat Bar Road Shifters, Shimano Compatible, Black : Sports & Outdoors
Shimano Ultegra ST-6510 2/3 x 9 Speed STI Shifter Brake Levers | eBay
Pre-owned: Shimano Ultegra (ST-6510) 9-speed right hand gear/brake lever – Cycloan Education And Restorative Approaches